How volatile sulfur compounds reshape soybean protein: From multiscale molecular binding mechanism study to emulsion system design

This study investigated the multiscale interaction between soy protein isolate (SPI) and key volatile sulfur compounds (VSCs)—dimethyl disulfide (DMD), dimethyl trisulfide (DMT), and lenthionine (LEN)—and its functional effects in emulsion systems. Multispectral analysis and molecular docking demonstrated that VSCs primarily bind to SPI via hydrophobic interactions and hydrogen bonds. DMD, DMT, and LEN exhibited binding affinities (ΔG) of −9.01, −10.40, and −11.97 kcal/mol, respectively. Due to its distinctive cyclic conformation, LEN formed two hydrogen bonds with ASN150 and LEU152 residues, showing the strongest binding. This binding induced conformational modifications in SPI, resulting in a reduction in α-helix content (from 14.9 % to 10.5 %) and an increase in β-sheet structure (from 23.6 % to 32.3 %) following LEN interaction, indicative of partial unfolding and aggregation. Microstructural analysis corroborated VSC-driven aggregation. LEN caused the most significant changes: a 100.9 % increase in particle size and an 89.00 % decrease in surface hydrophobicity, which matched the formation of irregular aggregates seen with atomic force microscopy (AFM). In SPI-stabilized oil-in-water emulsions, the binding strength of VSCs followed the order LEN > DMT > DMD, as indicated by flavor retention measurements. Furthermore, VSCs reinforced the interfacial protein layer via hydrophobic interactions, substantially enhancing the emulsion activity index (EAI) and the stability index (ESI). Headspace-gas chromatography-ion mobility spectrometry (HS-GC-IMS) revealed that VSCs, particularly LEN, influenced flavor release by decreasing off-flavor compounds such as hexanal and 1-octen-3-one, while promoting the release of agreeable esters, including ethyl acetate and ethyl propanoate. This research provides a mechanistic foundation for designing flavor-specific functional foods, elucidating the dual role of VSCs as both flavor modulators and stabilizers within plant-based emulsion systems.